1. Field of the Invention
The invention relates to a compound having a benzyl ester moiety in the minor axis direction of the molecule, a polymerizable liquid crystal composition including the compound, an anisotropic polymer formed from the polymerizable liquid crystal composition, a polymer film and their use.
2. Related Art
A polymerizable compound having a liquid crystal phase gives a polymer having a function such as optical compensation by polymerization. This is because the orientation of liquid crystal molecules is fixed by polymerization. A variety of polymerizable compounds have been studied in order to utilize such a function. However, it does not seem that one polymerizable compound have a sufficient function. Thus, the method has been tried in which a composition is prepared from some polymerizable compounds and the composition is polymerized (see the patent documents Nos. 1 and 2).
When liquid crystal molecules exhibit orientation such as homogeneous, tilted, homeotropic and twisted in this specification, they may be expressed as “having a homogeneous orientation”, “having a tilted orientation”, “having a homeotropic orientation”, and “having a twisted orientation”, respectively. For example, a liquid crystal film with a homogeneous molecular orientation, that is to say, a liquid crystal film that is oriented homogeneously, may be referred to as a liquid crystal film having a homogeneous orientation or a liquid crystal film of a homogeneous orientation.
A polymer having a homogeneous orientation can be used by way of a combination with a half-wave plate, a quarter-wave plate or a film having other optical functions (see the patent document No. 3). For the usage described above, a polymerizable liquid crystal material may sometimes be cumulated on a glass substrate, a glass substrate whose surface is covered with a plastic thin film (for example, an over-coated film formed on a color filter), a color filter-substrate (see the patent document No. 4) or a plastic substrate. Examples of a material used as a plastic substrate include polymers of TAC (triacetyl cellulose), polycarbonate, PET and cycloolefin-based resins.
The present inventors have found a polymerizable liquid crystal composition that maintains a liquid crystal phase stably also at room temperature and exhibits a uniform orientation (see the patent document No. 5). However, unresolved issues remain in which the liquid crystal phase cannot always be maintained when the composition ratio is changed in order to decrease the optical anisotropy (Δn). The addition of a compound having an aromatic ring (a compound having a triptycene ring) in the minor axis direction has been proposed as a method for adjusting the optical anisotropy. Even in this method, there are cases where crystallization occurs when a large amount of the compound is added to the composition, and unresolved issues remain with regard to the method for adjusting the optical anisotropy (see the patent document No. 6).
The present inventors found that a uniform orientation of the anisotropic polymer can be attained and the optical anisotropy can effectively be adjusted, when a polymerizable liquid crystal compound having a benzyl ester moiety in the minor axis direction of the molecule was used as a component of a polymerizable liquid crystal composition. They also found that the optical anisotropy can be decreased by a combination with a polymerizable triptycene derivative. Thus, they have completed the invention. The polymerizable liquid crystal compound having a benzyl ester moiety in the minor axis direction has at least one polymerizable group, and is polymerizable in the same manner as that of the other polymerizable liquid crystal compounds. An anisotropic polymer in which orientation is adjusted uniformly is formed, when this polymerizable liquid crystal composition is applied to a supporting substrate that has been processed with mechanical surface treatment such as rubbing, photo-alignment treatment or chemical surface treatment, and then the composition is polymerized.